Comparison and Evaluation Between MODIS Vegetation Indices in Northwest China

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组间和组内比较结果的英文注释

组间和组内比较结果的英文注释Between-Group and Within-Group Comparisons.In an analysis of variance (ANOVA), the researcher partitions the variability of a quantitative dependent variable into two sources: between-group and within-group variability.Between-Group Variability.Between-group variability refers to the variability in the dependent variable that is due to differences between the groups being compared. This variability is captured by the sum of squares between groups (SSbetween), which is calculated by subtracting the sum of squares within groups (SSwithin) from the total sum of squares (SStotal).The between-group variability can be further broken down into the variability due to the main effects of each independent variable and the variability due to theinteraction effects between the independent variables.Within-Group Variability.Within-group variability refers to the variability inthe dependent variable that is within each group being compared. This variability is captured by the sum ofsquares within groups (SSwithin), which is calculated by subtracting the sum of squares between groups (SSbetween) from the total sum of squares (SStotal).The within-group variability can be further broken down into the variability due to random error and thevariability due to individual differences within each group.Comparing Between-Group and Within-Group Variability.The ratio of between-group variability to within-group variability is known as the F-ratio. The F-ratio is used to test the statistical significance of the differences between the groups being compared.If the F-ratio is statistically significant, then it can be concluded that there is a significant difference in the dependent variable between the groups being compared.Post-Hoc Tests.If the overall ANOVA is statistically significant, then post-hoc tests can be used to determine which specific groups are different from each other. Post-hoc tests are multiple comparison procedures that control for the increased risk of Type I error that is associated with making multiple comparisons.Some of the most common post-hoc tests include:Tukey's HSD test.Scheffé's test.Bonferroni correction.Assumptions of ANOVA.The following assumptions must be met in order for ANOVA to be valid:The dependent variable is normally distributed.The variances of the dependent variable are equal across all groups.The groups are independent of each other.If these assumptions are not met, then the results of the ANOVA may be biased.Example.Suppose a researcher wants to compare the mean test scores of three different groups of students. The three groups are:Group A: Students who studied for the test for 1 hour.Group B: Students who studied for the test for 2 hours.Group C: Students who did not study for the test.The researcher conducts an ANOVA and finds that the F-ratio is statistically significant. This means that thereis a significant difference in the mean test scores of the three groups.The researcher then conducts post-hoc tests todetermine which specific groups are different from each other. The post-hoc tests show that Group A and Group Bhave significantly higher mean test scores than Group C. There is no significant difference in the mean test scoresof Group A and Group B.Conclusion.ANOVA is a powerful statistical technique that can be used to compare the means of two or more groups. By understanding the concepts of between-group and within-group variability, researchers can use ANOVA to gaininsights into the relationships between different variables.。

二甲基硅氧烷基的环氧树脂,被用来制造导热复合材料和12微米的BN颗

CERAMICSINTERNATIONALAvailable online at Ceramics International 40(2014)2047–2056Chemically modi ﬁed boron nitride-epoxy terminated dimethylsiloxanecomposite for improving the thermal conductivityKiho Kim,Myeongjin Kim,Yongseon Hwang,Jooheon Kim nSchool of Chemical Engineering &Materials Science,Chung-Ang University,Seoul 156-756,KoreaReceived 30May 2013;received in revised form 22July 2013;accepted 24July 2013Available online 21August 2013AbstractThe thermal conductivities of composites with an epoxy-terminated dimethylsiloxane (ETDS)matrix and boron nitride (BN)powder ﬁllers were investigated.Two surface curing agents,3-glycidoxypropyltrimethoxysilane (KBM-403)and 3-chloropropyltrimethoxysilane (KBM-703),were doped onto the surfaces of hydroxyl-functionalized boron nitride using a simple sol –gel process to act as ﬁllers in the thermally conducting composites.These synthesized materials were embedded in epoxy resin via a solvent casting method.The surface modi ﬁcation had an appreciable effect on the thermal conductivity resulting in increased thermal conductivity up to 70wt%.The thermal conductivities of the composites containing 70wt%BN particles treated with the KBM-403and KBM-703curing agents were 4.11and 3.88W/mK,respectively,compared to 2.92W/mK for the composite without surface treatment.&2013Elsevier Ltd and Techna Group S.r.l.All rights reserved.Keywords:posites;C.Thermal conductivity;D.Nitrides;Surface treatment1.IntroductionDemands for the miniaturization and continuous perfor-mance improvements of electronic packages have led to the development of new microelectronic packaging techniques.The typical characteristics of future microelectronic packaging include high density,high frequency,and high speed [1–3].It is well known that the reliability of an electronic device is exponentially dependent on the operating temperature of the junction,whereby a small difference of the operation tempera-ture can result in a two-fold reduction of the lifespan of a device [4].Improvements of size and performance are likely to result in the generation of a greater amount of heat in a smaller volume of space.To ensure proper device operation,the unwanted heat must be removed as quickly and effectively as possible to maintain the operation temperature,suggesting that the packaging materials of the product needs to have good thermal conductivity [5,6].Epoxy resin has been widely applied in electronics,paints,electrical insulators,printed circuit boards,and packaging materials as a matrix [7,8].Unfortunately,epoxy used alone as an electronic packaging material cannot effectively dissipate the heat generated from high packing and power-density devices,given the relatively low thermal conductivity range of epoxy of 0.1–0.3W/mK.Several studies have been conducted to improve the thermal conductivity of epoxy.For example,polymers ﬁlled with thermally conductive ﬁllers are emerging as a cost-effective means of coping with thermal management issues.Many researchers have investigated the thermal conductivity enhancement of composite materials including oxides (Al 2O 3,SiO 2,ZnO)[9–11],carbide (SiC)[12],and nitrides (AlN,BN,Si 3N 4)[13,14].The properties of these epoxy/inorganic ﬁller composites depend on the nature of the inorganic ﬁller including their chemical and physical composition,size,shape and dispersion in the epoxy matrix.Thermally conductive ﬁllers,like those mentioned above,have attracted attention due to their high thermal and low electrical conductivities,while metallic particles have high thermal and electrical conductiv-ities.Among the ﬁllers,boron nitride has been considered as/locate/ceramint0272-8842/$-see front matter &2013Elsevier Ltd and Techna Group S.r.l.All rights reserved./10.1016/j.ceramint.2013.07.117nCorresponding author.Tel.:þ8228205763.E-mail address:jooheonkim@cau.ac.kr (J.Kim).an attractive candidate due to its signiﬁcantly high thermal conductivity($300W/mK),lack of toxicity,superior chemi-cal stability,electrical insulation properties,and relatively low cost.In order to obtain a higher thermal conductivity while maintaining the existing properties,ﬁller loadings of60wt% or higher have traditionally been incorporated to form a heat conduction path in the matrix that is as continuous as possible. However,a highﬁller content becomes rather inﬂexible with voids and cracks forming betweenﬁllers.Moreover,the BN/ epoxy slurry has a relatively low viscosity before drying, which facilitates the sedimentation of the BNﬁller.As a result, BNﬁllers settle to the bottom of the compositeﬁlm,the heat-conductive path is cut off,and the thermal conductivity decreases.In response to this issue,many studies have suggested fabrication of a surface-modiﬁed BN particle to preventﬁller sedimentation[15,16].Recently,BN has been coated with a surface-curing agent to further enhance the thermal conductivity of polymer compo-sites,resulting in a reduction of sedimentation due to the lone-pair electron interaction of particle surfaces into the matrix.Gu et al.achieved a thermal conductivity of1.052W/mK using a hot disk instrument for epoxyﬁlled with silane-modiﬁed BN at a solid loading of60wt%[17].Yung et al.reported the effect of multi-modal particle size mixing on the formation of a thermally conductive network[18].Unfortunately,these previous experiments used nano-scale BN particles($1μm). Due to the small size of theﬁller-containedﬁlm,heat passing through the boundary of the particles is more frequently the main cause of the phonon scattering.The diffuse boundary scattering due to the short wavelength in comparison to interface roughness of dominant phonon heat carriers not only reduces the phonon mean free path,but can also destroy the coherence of the phonons[19].Thermal resistance at particle junctions known as thermal boundary resistance or Kapitza resistance is one of the primary causes of heat transfer property reduction.In the presence of a heatﬂux across the boundary,this thermal resistance causes a temperature dis-continuity at the boundary.Due to the differences of the electronic and vibrational properties of various materials,an energy carrier will scatter when attempting to traverse the interface[20–22].It is known that hexagonal BN particles have a plate-like shape withﬂat surfaces corresponding to the basal planes of the hexagonal crystal structure.The basal plane of BN is molecularly smooth and has no surface functional groups available for chemical bonding or interactions.In contrast,the edge planes of the platelets have functional groups such as hydroxyl and amino groups.These functional groups allow the BNﬁller to disperse in an organic solvent and chemically bond with other rge BN particles are signiﬁcantly decreased in the edge plane areas,resulting in difﬁculty obtaining uniform dispersion and chemical bonding.To this end,Sato et al.employed0.7μm BN particles as a thermal conductiveﬁller on polyimide resin[23,24].For the effective use of micro-scale BN as aﬁller,another surface treatment is necessary to make the reaction site of the particle surface.In this study,a polymer matrix,dimethyl siloxane-basedepoxy,was employed to fabricate a thermally conductivecomposite and a12μm BN particle was adopted as acompositeﬁller to reduce phonon scattering and improve thethermal conductivity.To increase the afﬁnity and dispersibilityof theﬁller in the epoxy matrix,which is expected to provide agood thermal conductivity,two types of surface curing agentswere employed.The resulting surface curing agent-coated BNpossessed electrical insulating properties in addition toenhanced thermal conductivity due to reduced thermal resis-tance at the junction.The mechanical properties of thefabricated composites were measured and the data demon-strates that a small amount of surface curing agent enhancesthe thermal and mechanical properties.2.Experimental2.1.Synthesis of ETDSThe epoxy-terminated dimethysiloxane(ETDS)oligomerwas obtained from Shin-Etsu silicon(KF-105,equivalentweight(E.E.W)=490g/eq,density=0.99g/cm3).In ourprevious study,the weight ratio of the epoxy to the curingagent was determined to provide efﬁcientﬂexibility of thematrix.In this study,the equivalent weight ratio of ETDS toDDM(4.4′-diamino diphenylmethane)was1:2.1.9g of DDMwas placed in a four-neck roundﬂask equipped with a reﬂuxcondenser and was preheated to363K.9.5g of the ETDSresin was added and heated in an oil bath at363K for1hunder a N2atmosphere.The bubbles in the mixture wereremoved by placing the mixture in a vacuum oven for30minat room temperature.The mixture was then placed in an oilbath at323K for10min in a N2atmosphere.Theﬁnaldegassing was performed in a vacuum oven for1h at roomtemperature to remove air bubbles[25].2.2.Surface modiﬁcation of BNThe detailed synthesis procedure is shown in Fig.1.First,micro-BN particles were suspended in a5M sodium hydro-xide solution at1101C for18h to attach more hydroxide ionsonto the surfaces.Because micro-BN particles have fewfunctional groups,surface treatment is necessary to facilitatechemical bonding with the surface curing agent.After basesolution dipping,the particles were rinsed with D.I.water and ﬁltered several times to adjust the pH from basic to neutral. The micro-BN hydroxide particles were left in the furnace at801C for5h,cooled to room temperature,and then stored indesiccators.The BN particles were modiﬁed with two surface curingagents,KBM-403and KBM-703,obtained from Shin-EtsuSilicon by a sol-gel reaction.An appropriate amount of3-glycidoxypropyltrimethoxysilane(KBM-403;3–5%based onthe weight of the micro-BN particles)and3-chlorop-ropyltrimethoxysilane(KBM-703)were added to D.I.waterand stirred at501C for30min to achieve hydrolysis.Themicro-BN hydroxide particles were then dipped into theK.Kim et al./Ceramics International40(2014)2047–2056 2048resulting solution and stirred at 701C for 1h,followed by rinsing with D.I.water and ﬁltering three times.The particles were then vacuum dried at 801C for 5h to remove the solvent.The amount of the coating solution necessary is preferably 0.05–10%by weight,based on the weight of the particles.When the amount of the coating solution is less than 0.05%by weight,there is a tendency for insuf ﬁcient and non-uniform particle coating to occur.When the amount of the coating solution exceeds 10%by weight,the obtained particles may possess excessive thermal resistance,thereby causing a decrease of the thermal conductivity of the composite ﬁlms [26].2.3.Preparation of the BN/epoxy compositesThe composites were prepared by solution blending and a casting method consisting of (a)adding surface curing agent-coated micro-BN to the ETDS epoxy resin (50,60,70wt%)for approximately 3h in N,N-dimethylformamide (DMF)until the synthesized materials were completely mixed,(b)fabricat-ing the composite ﬁlms to a uniform thickness via a doctor blade on the Te ﬂon mold,(c)pre-curing the ﬁlms at 1501C for 3h until no air bubbles appeared on the surface followed by post-curing at 1801C for 5h,and (d)cooling to room temperature.2.4.CharacterizationFourier transform infrared (FT-IR;Parkin-Elmer Spectrum One)spectroscopy and X-ray photoelectron spectroscopy (XPS;VG-Microtech,ESCA2000)were employed to analyze the surface curing agent-coated BN.For FT-IR spectroscopy,the ATR mode was used to avoid the in ﬂuences of moisture adsorbed on the potassium bromide (KBr)particles and the scans were performed using radiation in the frequency range of 400–4000cm À1.In the XPS analysis,a monochromatic Mg K αX-ray source was used at 1253.6eV and the Gaussian peak widths obtained by curve ﬁtting were constant in each spectrum.Thermogravimetric analysis (TGA;TGA-2050,TAinstrument)of the samples was carried out to examine the thermal degradation of BN,BN-403,and BN-703.4mg of the samples were heated to 8001C at a heating rate of 101C/min under a nitrogen atmosphere.Field emission scanning electron microscopy (FE-SEM;Sigma,Carl Zeiss)was carried out to con ﬁrm the cross-sections of the component ﬁlms before and after the silane treatment.The samples were sputtered with a thin layer of platinum to avoid the accumulation of charge before the FE-SEM observations.The thermal diffusivity (δ,mm 2s À1)at room temperature was measured on disk samples using a laser ﬂash method (Netzsch Instruments Co.,Nano ﬂash LFA 447System).The speci ﬁc heat (C ,J g À1K À1)at room temperature was measured on disk samples via differential scanning calorimetry (DSC;Perkin-Elmer Co.,DSC-7System)and the bulk density (ρcomp ,g cm À3)of the specimens was measured using the water displacement method.The thermal conductivity (Ф,W mK À1)was calcu-lated using the following equation:Ф¼δC ρcompTo study the mechanical properties of the composite materials,mechanical analysis (DMA;Triton Instrument,Triton DMTA)was carried out.The storage modulus of the solid ﬁlms was measured at a frequency of 1Hz.The temperature range was À180to 1801C with cooling and heating rates of 101C/min.3.Results and discussion3.1.Structure analysisThe chemical structures of the surface-modi ﬁed BN were determined using Fourier transform infrared spectroscopy (FT-IR),thermogravimetric analysis (TGA),and X-ray photo-electron spectroscopy (XPS).Fig.2(a –c)shows the FT-IR spectra of pristine BN,BN-403,and BN-703,respectively.For pristine BN,the bands at 1400and 800cm À1indicate stretching vibration in the hexagonal BN.The absorption band of pristine BN at 2300–2380cm À1represents absorbed CO 2.Fig.1.Reaction scheme for the preparation of surface curing agent treated BN particle (BN-403,BN-703).K.Kim et al./Ceramics International 40(2014)2047–20562049Comparing the pristine BN and the surface curing agent-treated BN (BN-403and BN-703),the new band at 1100cm À1corresponds to stretching of the Si –O bonds,which existed in the pure surface curing agent.Detailed surface information of pristine BN,BN –OH,BN-403,and BN-703was collected by X-ray photoelectron spectroscopy (XPS)and the corresponding results are pre-sented in Fig.3.In the spectrum of pristine BN,there are only two elements of B and N.However,the O 1s signal emerges in the spectrum of BN –OH.This result implies that the hydroxyl groups were effectively introduced on the BN surfaces and edges via the sodium hydroxide treatment.These hydroxyl groups,acting as anchor sites,enabled attachment between the BN particles and surface curing agent.Moreover,the new peaks of C 1s and Si 2p can be assigned in the spectra of both BN-403and BN-703,indicating that both surface curing agents,403and 703,were successfully attached to the surface and edges of pristine BN particles.The detailed chemical bonding of fabricated,surface-treated BN particles was con ﬁrmed from the de-convoluted B 1s,Si 2p,and C 1s spectra,the results of which are shown in Fig.4.Fig.4(a and b)show the de-convoluted B 1s spectra of BN-403and BN-703,respectively.The B 1s spectra of both BN-403and BN-703showed a strong binding energy peak for the B –N bond and a weak binding energy peak for the B –OH bond at 190.4eV and 192eV,respectively [27,28].The B –OH peak resulted from the introduction of a hydroxyl group by the base treatment.In order to provide clearer evidence of chemical bonding between the BN particles and the silane curing agent,the Si 2p peak of these synthesized materials can be ﬁtted by a curve with several component peaks.Fig.4(c and d)shows the de-convoluted Si 2p spectra of BN-403and BN-703,respectively.In the spectra of both BN-403and BN-703,the strong peak at the binding energy of 102.1eV represents the bond between silicon and oxygen originating from the BN particles (B –O –Si),indicating that the surface curing agent and BN particles are connected through the hydroxyl groups.The peak at 103.3eV is attributed to siloxane (Si –O –Si)resulting from the partial hydrolysis of the silane curing agent molecules during the silanization reaction.Moreover,the peak at 100.8eV is attributed to Si –C bonding in the silane curing agent molecules.These results are in agreement with the reaction mechanism of silane,including the hydrolysis of –OCH 3,condensation to oligomers,hydrogen bonds between oligomer and hydroxyl groups on the substrate,and the formation of the covalent linkage between silane and the substrate.The peak at 102.5eV is attributed to Si –OH bonding,indicating that some hydroxyl groups did not hydro-lyze and a small amount of hydroxyl group remained [29,30].This implies that the amount of hydroxyl groups on the BN surface was not suf ﬁcient to make a uniform siloxane network and some hydroxyl groups in the surface curing agent were not hydrolyzed.This is due to the basal plane of the boron nitride particle surface having no functional groups.In addition,the silane coupling agent does not coat the surface uniformly such that the hydroxyl groups of the surface curing agent remained.The C 1s spectra of both BN-403and BN-703were de-convoluted to compare the structural differences of the two silane curing agents and the results are shown in Fig.4(e and f),respectively.In the spectra of both BN-403and BN-703,the strong peak at a binding energy of 284.7eV indicates the C –C bond and the weak peak at the binding energy of 283.44eV represents the C –Si bond,which exists in the surface curing agent structure.The primary difference between KBM-403and KBM-703is that KBM-403has ether and epoxide groups,whereas KBM-703has a chloride atom at the end of the carbon chain.In the C 1s spectrum of BN-403,C –O bonding is observed at a binding energy of 286.2eV,which originates from the ether and epoxide groups.However,in the case of BN-703,the peak at 285.9eV is attributed to C –Cl bonding,which can be explained by the existence of a chloride atom at the end of the carbon chain [31].Based on these results,it can be concluded that the silane treatment can effectively introduce the surface curing agent onto the surface ofBN.Fig.2.FT-IR spectra of (a)raw BN,(b)BN-403and (c)BN-703.Fig.3.X-ray photoelectron spectroscopy survey scans of (1)raw BN,(b)BN-OH,(c)BN-403,and (d)BN-703.K.Kim et al./Ceramics International 40(2014)2047–20562050Fig.4.XPS spectra of BN-403and BN-703.(a)XPS B 1s spectrum of BN-403:(b)XPS B 1s spectrum of BN-703:(c)XPS Si 2p spectrum of BN-403:(d)XPS Si 2p spectrum of BN-703;(e)XPS C 1s spectrum of BN-403:(f)XPS C 1s spectrum of BN-703.K.Kim et al./Ceramics International 40(2014)2047–20562051The compositions of the as-prepared BN,BN-403,and BN-703composites were further investigated via TGA (Fig.5).The experiments were performed up to 8001C in air at a heating rate of 101C min À1.Under these conditions,weight loss was not observed up to 8001C for pristine BN,whereas weight losses of about 4%and 3.6%were observed for BN-403and BN-703,respectively,due to thermal decomposition of the surface curing agents attached to the BN.The mass ratios of KBM-403/BN and KBM-703/BN for BN-403and BN-703were calculated to be 0.042/1and 0.037/1,respec-tively.Moreover,these ratios between BN and the surface curing agent are optimal to mix the surface curing agent-treated BN as a ﬁller and the silane-based epoxy as a matrix [32].As reported by Itoh et al.,the surface curing agent is poorly dispersed in the particles when too little is added and the desired effect of improved crack formation resistance in the cured resin composition cannot be achieved [33].An excess of surface curing agent in the resin composition leads to a decreased thermal conductivity.This is because the redundant coupling agent causes phonon scattering and gives rise to a decreased thermal conductivity of the composites,resulting in low thermal conductivity materials.3.2.Thermal propertiesThe thermal conductivity of the composites is controlled by the intrinsic conductivities of the ﬁller and matrix as well asthe shape,size,and loading level of the ﬁller.Table 1shows the variations of the thermal conductivity of the BN/ETDS composites with nano-BN ($1μm),micro-BN (8μm,12μm),and a ﬁller content ranging from 50to 70wt%.The thermal conductivity of pristine ETDS is approximately 0.2W/mK.It can be seen that as the weight fraction of these ﬁllers increased,the thermal conductivity also increased consider-ably.With 12μm particles at loadings of 50wt%,60wt%,and 70wt%,the thermal conductivity increased by factors of 11.1,13.35,and 14.58,respectively,compared to the pure resin ($0.2W/mK).As shown for the three types of particles,the thermal conductivity is also a function of the particle size,where the results parallel the effect of the particle type.The highest thermal conductivity values were obtained from the 12μm particle-ﬁlled composites at all ﬁller loading levels.This result can be explained by the thermal interface resistance caused by phonon boundary scattering.In theory,the scatter-ing of phonons in composite materials is primarily due to the existence of an interfacial thermal barrier from acoustic mismatch or damage of the surface layer between the ﬁller and the rge particles tend to form fewer thermally resistant polymer-layer junctions than small particles at the same ﬁller rge particles are therefore used as a thermal conducting ﬁller because of their negligible phonon scattering effect and excellent thermal conductivity [34,35].In this paper,the use of a micro-ﬁller to improve the thermal conductivity of composites was studied.The in ﬂuence of the BN concentration and surface treatment on the thermal conductivity of BN/ETDS composites is presented in Fig.6.It can be seen that as the weight fraction of these synthesized materials increased,the calculated thermal conductivity of all of the investigated composites increased considerably.The use of surface curing agents clearly improved the thermal con-ductivities of the composites.At 12μm BN particle loadings of 50,60,and 70wt%,the thermal conductivities of the KBM-403-and KBM-703-treated BN/ETDS composites increased by factors of 1.17,1.23,and 1.41and 1.15,1.19,and 1.33compared to the untreated BN/ETDS composite,respectively.This effect could be explained by the enhanced dispersibility of particles in the composite caused by the surface curing agent.The two surface curing agents used contain an epoxide group and chloride functional groups that interact with the active groups of the epoxy matrix.Thus,the organic active groups or long molecular chains on the surface of the modi ﬁed BN either react or entangle with the reactive groups of the epoxy matrix.The addition of surface curing agents totheFig.5.TGA thermograms of pristine BN,BN-403and BN-703.Table 1Thermal conductivity of various particle size and ﬁller contents (W/mK).Size of BN particlesThermal conductivity at various ﬁller concentration [W/mK]50wt%60wt%70wt%$1m m 1.49 1.67 2.158m m 2.16 2.35 2.7712m m2.322.732.92K.Kim et al./Ceramics International 40(2014)2047–20562052epoxy matrix therefore improves the interface bonding between the BN particles and matrix,leading to an enhanced thermal conductivity.In addition,this result could beexplained by the reduction of the phonon diffuse boundary scattering that constitutes a signi ﬁcant part of the thermal resistance accompanying an imposed temperature gradient.The phonon scattering at interfaces,both for free surfaces and those bonded to other materials,observed for most real interfaces has yet to be quantitatively explained.The boundary resistance between two carefully bonded solids appears to be satisfactorily described by the acoustic impedance mismatch between the two media.The silane coupling agents acted as phonon transfer bridges between the polymer and the ceramic ﬁller,which reduced the phonon boundary scattering and improved the thermal conductivity at low concentrations.As demonstrated in this study,the KBM-403treatment was more effective than KBM-703in enhancing the thermal conductivity.This can be explained by the ETDS/DDM polymerization mechanism.When the composite curing was performed at high temperature,epoxide groups in the ETDS react with DDM,and the ring opening and polymerization reactions proceed continuously.Similarly,epoxide groups in KBM-403react with DDM and are polymerized with ETDS.As a result,the boron nitride ﬁller linked with the matrix through covalent bonding.However,KBM-703has a chloride group that instead forms non-covalent bonding,a dipole-dipole interaction,with the ETDS matrix.Therefore,KBM-403-Fig.6.Effect of surface treatment of BN particles on the thermal conductivity of BN/ETDS composites at various ﬁllerconcentration.Fig.7.SEM cross section image of (a)–(b)BN/ETDS,(c)–(d)BN-403/ETDS and (e)–(f)BN-703/ETDS composites with 50wt%ﬁller concentration.K.Kim et al./Ceramics International 40(2014)2047–20562053treated BN has a stronger interaction with the ETDS matrix along with good dispersibility and a higher thermal conduc-tivity than the KBM-703-treated BN composite.The differences in the cross-sectional images of each composite can be correlated to the existence of a silane coupling agent.Fig.7shows FE-SEM images of the cross-sections of50wt%BN/ETDS and two kinds of surface coupling agent-treated BN/ETDS compositeﬁlms.As observed in Fig.7(a,c and e),all of the compositeﬁlms appear to be homogeneously distributed.However,when the BN/epoxy compositeﬁlm is observed at higher magniﬁcation (Fig.7(b)),it can be seen that the BNﬁller settled to the bottom and a non-uniform distribution can be observed in the top of theﬁlm.This phenomenon is due to sedimentation of theﬁller,which is an endemic problem of the casting method. BNﬁllers were well mixed for a sufﬁcient time with the epoxy resin,maintaining goodﬂuidity at the high temperature,but sedimentation of theﬁller progressed in the post curing step. On the other hand,the images in Fig.7(d and e)show a uniform cross-section of the silane coupling agent-treated BN, which was dispersed more uniformly and embedded in theepoxy,creating superior interface adhesion with the BN in the epoxy matrix.Uniform distribution of BN particles develops in the conduction carrier path.Moreover,BN has an idiopathic high surface energy,indicating that the phase interaction force between the epoxy and BN particles is very weak and suggesting that a low energy is needed to pull BN particles out from the matrix[36].The images in Fig.7(d and f)show an enhanced homogeneous distribution of BN particles throughout and a decrease of cracks and voids between the BN particles.This is further evidence that the surface curing agent enhanced the BN particle afﬁnity with the matrix. Fig.7shows that the use of surface coupling agents effectively improves the homogeneous dispersion of BN particles in the epoxy,eliminates the agglomeration ofﬁller, and decreases the void content and defects in the composites, resulting in an increased thermal conductivity.3.3.Mechanical propertiesThe mechanical properties of the composites with enhanced thermal conductivities were also measured by dynamic mechanical analysis(DMA).This was carried out to determine the improvement of the mechanical properties after the surface modiﬁcation of the boron nitride particles in the polymer matrix.Since the operation temperature of the electronic package is generally about1501C,the stability of the composites was examined up to this temperature.The storage modulus of compositeﬁlms with aﬁller content of70wt%is displayed in Fig.8as a function of temperature.As expected, the slopes of the curves tended to decrease near the glass temperature(T g).It can be clearly seen that the storage modulus of the composites increased withﬁller surface modiﬁcation,which is due to the mechanical reinforcement resulting from the strong interactions between BN and the ETDS matrix.As mentioned above,silane coupling agents improved the adhesive property between theﬁller and matrix, as the stress is not well transferred when the same force is applied to the composite.The KBM-403in the composites prevented efﬁcient treeing of the propagation of stress and,as a result,the storage modulus could be improved where the modulus of the KBM-403treated with the BN composite was higher than that of the KBM-703-treated BN composite. The tanδpeak position,which is a measure of the glass transition temperature(T g),shifted to higher temperatures with surface modiﬁcation.The peak height was reduced when compared to that of the surface-untreated BN composite because the well dispersedﬁller and sufﬁcient incorporation of epoxy restricted the mobility of the epoxy chains,resulting in the higher mechanical properties observed for the surface-treated BN composites.4.ConclusionBN/epoxy compositeﬁlms with different BN particle sizes and contents were successfully fabricated with a surface curing agent using a solvent casting method.The thermal conductiv-ities of polymer compositesﬁlled with various types of particles were evaluated and the thermal interface resistance theory was applied.Various particle toﬁller ratios were tested and the12μmﬁller demonstrated a higher performance,in the range of136–149%,than the nano-sizeﬁller($1μm). Furthermore,by applying micro-sized particles,the formation of conductive networks was maximized while minimizing the thermal interface resistance along the heatﬂow path.This thermal interface resistance is caused by phonon scattering in the interface of materials,which is the primary cause of decreased thermalconductivity.Fig.8.Storage modulus of ETDS and ETDS composites with70wt%ﬁller concentration.K.Kim et al./Ceramics International40(2014)2047–2056 2054。

摘录不饱和聚酯文献中的经典句子

1.To access the description of a composite material, it will be necessary to specify the nature of components and their properties, the geometry of the reinforcement, its distribution, and the nature of the reinforcement–matrix interface.2. However, most of them are not chemically compatible with polymers3. That’s why for many years, studies have been conducted on particles functionalization to modulate the physical and/or chemical properties and to improve the compatibility between the ﬁller and the matrix [7].4. Silica is used in a wide range of products including tires, scratch-resistant coatings, toothpaste,medicine, microelectronics components or in the building5. Fracture surface of test specimens were observed by scanning electron microscopy6.Test specimens were prepared by the following method from a mixture composed with 40 wt% UPE, 60 wt% silica Millisil C6 and components of ‘‘Giral.’7.Grafted or adsorbed component amounts on modiﬁed silica samples were assessed by thermogravimetric analysis (TGA) using a TGA METTLER-TOLEDO 851e thermal system. For the analysis, about 10–20 mg of samples were taken and heated at a constant rate of 10 C/min under air (purge rate 50 mL/min) from 30 to 1,100 C.8.Nanocomposites with different concentrations of nanofibers wereproduced and tested, and their properties were compared with those of the neat resin.9.Basically, six different percentages were chosen, namely 0.1, 0.3, 0.5, 1, 2, 3 wt %.10.TEM images of cured blends were obtained with a Philips CM120 microscope applying an acceleration voltage of 80 kV.Percolation threshold of carbon nanotubes ﬁlled unsaturated polyesters 11.For further verification, the same experiment was carried out for the unmodified UP resin, and the results showed that there were no endothermic peaks12.The MUP resin was checked with d.s.c, scanning runs at a heating rate of 10°C min 1. Figure 4a shows that an endothermic peak appeared from 88 to 133°C, which indicates bond breaking in that temperature range.13.On the basis of these results, it is concluded that a thermally breakable bond has been introduced into the MUP resin and that the decomposition temperature is around I lO°C.14.The structures of the UP before and after modification were also checked with FTi.r. Figure 5 shows a comparison of the i.r. spectra of the unmodified and modified UP resins.15This is probably a result of the covalent bonding ofthe urethane linkage being stronger than the ionic bondingof MgO.16.These examples show that different viscosity profiles can be designed with different combinations of the resins and thickeners according to the needs of the applications.17. A small secondary reaction peak occurred at higher temperatures, probably owing to thermally induced polymerization. 18.Fiber-reinforced composite materials consist of ﬁbers of high strength and modulus embedded in or bonded to a matrix with a distinct interfaces between them.19.In this form, both ﬁbers and ma-trix retain their physical and chemical identities,yet they provide a combination of properties that cannot be achieved with either of the constituents acting alone.20.In general, ﬁbers are the principal load-bearing materials, while the surrounding matrix keep them in the desired location, and orientation acts as a load transfer medium between them and protects them from environmental damage.21.Moreover, both the properties, that is,strength and stiffness can be altered according to our requirement by altering the composition of a single ﬁber–resin combination.22.Again, ﬁber-ﬁlled composites ﬁnd uses in innumerable applied ar- eas by judicious selection of both ﬁber and resin.23.In recent years, greater emphasis has been rendered in the development of ﬁber-ﬁlled composites based on natural ﬁbers with a view to replace glass ﬁbers either solely or in part for various applications. 24.The main reasons of the failure are poor wettability and adhesion characteristics of the jute ﬁber towards many commercial synthetic resins, resulting in poor strength and stiffness of the composite as well as poor environmental resistance.25.Therefore, an attempt has been made to overcome the limitations of the jute ﬁber through its chemical modiﬁcation.26.Dynamic mechanical tests, in general, give more information about a composite material than other tests. Dynamic tests, over a wide range of temperature and frequency, are especially sensitive to all kinds of transitions and relaxation process of matrix resin and also to the morphology of the composites.27.Dynamic mechanical analysis (DMA) is a sensitive and versatile thermal analysis technique, which measures the modulus (stiffness) and damping properties (energy dissipation) of materials as the materials are deformed under periodic stress.28.he object of the present article is to study the effect of chemical modiﬁcation (cyanoethylation)of the jute ﬁber for improving its suitability as a reinforcing material in the unsaturated polyesterres in based composite by using a dynamic mechanical thermal analyzer.30.General purpose unsaturated polyester resin(USP) was obtained from M/S Ruia Chemicals Pvt. Ltd., which was based on orthophthalic anhydride, maleic anhydride, 1,2-propylene glycol,and styrene.The styrene content was about 35%.Laboratory reagentgrade acrylonitrile of S.D.Fine Chemicals was used in this study without further puriﬁcation. 31.Tensile and ﬂexural strength of the ﬁbers an d the cured resin were measured by Instron Universal Testing Machine (Model No. 4303).32.Test samples (60 3 11 3 3.2 mm) were cut from jute–polyester laminated sheets and were postcured at 110°C for 1 h and conditionedat 65% relative humidity (RH) at 25°C for 15 days.33.In DMA, the test specimen was clamped between the ends of two parallel arms, which are mounted on low-force ﬂexure pivots allowing motion only in the horizontal plane. The samples in a nitrogen atmosphere were measured in the ﬁxed frequency mode, at an operating frequency 1.0 HZ (oscillation amplitude of 0.2 mm) and a heating rate of 4°C per min. The samples were evaluated in the temperature range from 40 to 200°C.34.In the creep mode of DMA, the samples were stressed for 30 min at an initial temperature of 40°C and allowed to relax for 30 min. The tem- perature was then increased in the increments of 40°C, followed by an equilibrium period of 10min before the initiation of the next stress relax cycle. This program was continued until it reached the temperature of160°C. All the creep experiments were performed at stress level of20 KPa (approximate).35.The tensile fracture surfaces of the composite samples were studied with a scanning electron microscope (Hitachi Scanning electron Microscope, Model S-415 A) operated at 25 keV.36.The much im proved moduli of the ﬁve chemically modiﬁed jute–polyester composites might be due to the greater interfacial bond strength between the ma trix resin and the ﬁber.37.The hydrophilic nature of jute induces poor wettability and adhesion characteristics with USP resin, and the presence of moisture at the jute–resin interface promotes the formation of voids at the interface. 38.On the other hand, owing to cyanoethylation, the moisture regain capacity of the jute ﬁber is much reduced; also, the compatibility with unsaturated polyester resin has been improved and produces a strong interfacial bond with matrix resin and produces a much stiffer composite.39.Graphite nanosheets(GN), nanoscale conductive ﬁller has attracted signiﬁcant attention, due to its abundance in resource and advantage in forming conducting network in polymer matrix40.The percolation threshold is greatly aﬀected by the properties of the ﬁllers and the polymer matrices,processing met hods, temperature, and other related factors41.Preweighted unsaturated polyester resin and GN were mixed togetherand sonicated for 20 min to randomly disperse the inclusions.42.Their processing involves a radical polymerisation between a prepolymer that contains unsaturated groups and styrene that acts both asa diluent for the prepolymer and as a cross-linking agent.43.They are used, alone or in ﬁbre-reinforced composites, in naval constructions, offshore applications,water pipes, chemical containers, buildings construction, automotive, etc.44.Owing to the high aspect ratio of the ﬁllers, the mechanical, thermal, ﬂame retardant and barrier properties of polymers may be enhanced without a signiﬁcant loss of clarity, toughness or impact strength.45.The peak at 1724 cm-1was used as an internal reference, while the degree of conversion for C=C double bonds in the UP chain was determined from the peak at 1642 cm-1and the degree of conversion for styrene was calculated through the variation of the 992 cm-1peak46. Paramount to this scientific analysis is an understanding of the chemorheology of thermosets.47．Although UPR are used as organic coatings, they suffer from rigidity, low acid and alkali resistances and low adhesion with steel when cured with c onventional ‘‘small molecule’’ reagents.48．Improvements of resin ﬂexibility can be obtained by incorporating long chain aliphatic com-pounds into the chemical structure of UPR. 47．In this study, both UPR and hardeners were based on aliphatic andcycloaliphatic systems to produce cured UPR, which have good durability with excellent mechan-ical properties.50．UPR is one of the widely used thermoset polymers in polymeric composites, due to their good mechanical properties and relatively inexpensive prices.51．[文档可能无法思考全面，请浏览后下载，另外祝您生活愉快，工作顺利，万事如意!]。

经皮椎间孔镜与改良PLIF手术治疗老年性腰椎管狭窄症的疗效比较

doi:10.3969/j.issn.1009-881X.2020.03.004经皮椎间孔镜与改良PLIF手术治疗老年性腰椎管狭窄症的疗效比较黄群，朱现玮，严飞，徐炜，徐沁，周志平（苏州大学附属张家港医院，江苏张家港 215600）摘要：目的比较经皮椎间孔镜下减压术与改良后路减压融合内固定术在治疗老年性腰椎管狭窄症的临床疗效。

方法回顾分析78 例获得完整随访的老年性腰椎管狭窄症患者的治疗情况。

根据手术方法患者分为2组，开放手术组：40 例行改良后路减压融合内固定术（改良PLIF），微创手术组：38 例行经皮椎间孔镜下减压术。

记录两组患者围手术期观察指标、视觉疼痛模拟评分（visual analogue scale, V AS）、Oswestry功能障碍指数（Oswestry disability index, ODI）和改良Macnab 疗效评定标准对手术疗效进行评估。

结果微创手术组患者手术时间、切口长度、术中出血量、住院时间及术后并发症发生率明显优于开放手术组，差异具有统计学意义（P＜0.05）。

微创手术组手术优良率（89.47%）明显优于开放手术组（80.00%），差异具有统计学意义（P＜0.05）。

随访结果显示微创手术组术后各时间段V AS评分、ODI评分均优于开放手术组，差异具有统计学意义（P＜0.05）。

术后1 年两组患者VAS评分、ODI评分差异无统计学意义（P＞0.05）。

结论对于老年性腰椎管狭窄症患者，经皮椎间孔镜技术能够取得与改良PLIF手术相似的临床效果，但经皮椎间孔镜技术在手术时间、切口长度、术中出血量、住院时间、术后并发症等方面具有明显优势，并且对脊柱稳定性影响较小，症状改善明显，术后恢复较快，疗效确切。

但在临床应用上，需要严格把握手术适应证。

关键词：腰椎管狭窄症；经皮椎间孔镜；后路减压融合内固定；老年中图分类号：R687.3 文献标识码：A 文章编号：1009-881X(2020)03-0287-06 Comparison of the Efficacy of Percutaneous Interforamoscopy and Modified PLIF in the Treatment of Senile Lumbar Spinal StenosisHUANG Qun, ZHU Xian-wei, YAN Fei, XU Wei, XU Qin, ZHOU Zhi-ping(Zhangjiagang Hospital Affiliated to Soochow University, Zhangjiagang, Jiangsu, 215600, China) Abstract：Objective To compare the clinical efficacy of percutaneous endoscopic discectomy and posterior decompression fusion internal ﬁxation in the treatment of senile lumbar spinal stenosis. Methods A retrospective analysis was made on the treatment of 78 cases of senile lumbar spinal stenosis with complete follow-up. The patients were divided into 2 groups according to the surgical method. In open surgery group, 40 patients underwent the posterior decompression fusion internal ﬁxation (improved PLIF); in minimally invasive surgery group, 38 patients underwent percutaneous endoscopic discectomy. Perioperative observation indicators, visual analogue scale (V AS), Oswestry disability index (ODI) and modiﬁed Macnab efficacy evaluation criteria were recorded to evaluate the surgical efficacy of the two groups. Results The operative time, incision length, intraoperative blood loss, length of stay and incidence of postoperative complications in the minimally invasive surgery group were signiﬁcantly better than those in the open surgery group, with statistically signiﬁcant diﬀerences (P＜0.05). The 基金项目：国家自然科学基金资助项目(81874008)收稿日期：2020-04-25 作者简介：黄群(1988—),男,江苏南通人,硕士,主治医师,研究方向:慢性脊柱退行性疾病。

八年级英语议论文论证方法单选题40题

八年级英语议论文论证方法单选题40题1. In the essay, the author mentions a story about a famous scientist to support his idea. This is an example of _____.A.analogyB.exampleparisonD.metaphor答案：B。

本题主要考查论证方法的辨析。

选项A“analogy”是类比；选项B“example”是举例；选项C“comparison”是比较；选项D“metaphor”是隐喻。

文中提到一个关于著名科学家的故事来支持观点，这是举例论证。

2. The writer uses the experience of his own life to prove his point. This kind of method is called _____.A.personal storyB.example givingC.case studyD.reference答案：B。

选项A“personal story”个人故事范围较窄；选项B“example giving”举例；选项C“case study”案例分析；选项D“reference”参考。

作者用自己的生活经历来证明观点，这是举例论证。

3. The author cites several historical events to strengthen his argument. What is this method?A.citing factsB.giving examplesC.making comparisonsing analogies答案：B。

选项A“citing facts”引用事实，历史事件可以作为例子，所以是举例论证；选项B“giving examples”举例；选项C“making comparisons”比较；选项D“using analogies”使用类比。

学术英语(理工)详解答案-Unit-5

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① A group of female rats fed by GM soya flour before
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Unit 5
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1th6at are discussed by scientists worldwide?
In developing countries, lost crops are only one cause of hunger. Poverty plays the largest role.
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Hale Waihona Puke 1 DefinitionMaEtcnhathnecinwgorydosuwr aitchatdhemir idcelfainigtiuoangse.
The author means that in fact, genetically modified foods are already very much a part of our lives.

英语文摘

Bull Earthquake Eng(2008)6:645–675DOI10.1007/s10518-008-9078-1ORIGINAL RESEARCH PAPERNumerical analyses of fault–foundation interactionI.Anastasopoulos·A.Callerio·M.F.Bransby·M.C.R.Davies·A.El Nahas·E.Faccioli·G.Gazetas·A.Masella·R.Paolucci·A.Pecker·E.RossignolReceived:22October2007/Accepted:14July2008/Published online:17September2008©Springer Science+Business Media B.V.2008Abstract Field evidence from recent earthquakes has shown that structures can be designed to survive major surface dislocations.This paper:(i)Describes three differentﬁnite element(FE)methods of analysis,that were developed to simulate dip slip fault rupture propagation through soil and its interaction with foundation–structure systems;(ii)Validates the developed FE methodologies against centrifuge model tests that were conducted at the University of Dundee,Scotland;and(iii)Utilises one of these analysis methods to conduct a short parametric study on the interaction of idealised2-and5-story residential structures lying on slab foundations subjected to normal fault rupture.The comparison between nume-rical and centrifuge model test results shows that reliable predictions can be achieved with reasonably sophisticated constitutive soil models that take account of soil softening after failure.A prerequisite is an adequately reﬁned FE mesh,combined with interface elements with tension cut-off between the soil and the structure.The results of the parametric study reveal that the increase of the surcharge load q of the structure leads to larger fault rupture diversion and“smoothing”of the settlement proﬁle,allowing reduction of its stressing.Soil compliance is shown to be beneﬁcial to the stressing of a structure.For a given soil depthH and imposed dislocation h,the rotation θof the structure is shown to be a function of:I.Anastasopoulos(B)·G.GazetasNational Technical University,Athens,Greecee-mail:ianast@civil.ntua.grA.Callerio·E.Faccioli·A.Masella·R.PaolucciStudio Geotecnico Italiano,Milan,ItalyM.F.BransbyUniversity of Auckland,Auckland,New ZealandM.C.R.Davies·A.El NahasUniversity of Dundee,Dundee,UKA.Pecker·E.RossignolGeodynamique et Structure,Paris,France123(a)its location relative to the fault rupture;(b)the surcharge load q;and(c)soil compliance.Keywords Fault rupture propagation·Soil–structure-interaction·Centrifuge model tests·Strip foundation1IntroductionNumerous cases of devastating effects of earthquake surface fault rupture on structures were observed in the1999earthquakes of Kocaeli,Düzce,and Chi-Chi.However,examples of satisfactory,even spectacular,performance of a variety of structures also emerged(Youd et al.2000;Erdik2001;Bray2001;Ural2001;Ulusay et al.2002;Pamuk et al.2005).In some cases the foundation and structure were quite strong and thus either forced the rupture to deviate or withstood the tectonic movements with some rigid-body rotation and translation but without damage(Anastasopoulos and Gazetas2007a,b;Faccioli et al.2008).In other cases structures were quite ductile and deformed without failing.Thus,the idea(Duncan and Lefebvre1973;Niccum et al.1976;Youd1989;Berill1983)that a structure can be designed to survive with minimal damage a surface fault rupture re-emerged.The work presented herein was motivated by the need to develop quantitative understan-ding of the interaction between a rupturing dip-slip(normal or reverse)fault and a variety of foundation types.In the framework of the QUAKER research project,an integrated approach was employed,comprising three interrelated steps:•Field studies(Anastasopoulos and Gazetas2007a;Faccioli et al.2008)of documented case histories motivated our investigation and offered material for calibration of the theoretical methods and analyses,•Carefully controlled geotechnical centrifuge model tests(Bransby et al.2008a,b)hel-ped in developing an improved understanding of mechanisms and in acquiring a reliable experimental data base for validating the theoretical simulations,and•Analytical numerical methods calibrated against the aboveﬁeld and experimental data offered additional insight into the nature of the interaction,and were used in developing parametric results and design aids.This paper summarises the methods and the results of the third step.More speciﬁcally: (i)Three differentﬁnite element(FE)analysis methods are presented and calibratedthrough available soil data.(ii)The three FE analysis methods are validated against four centrifuge experiments con-ducted at the University of Dundee,Scotland.Two experiments are used as a benchmark for the“free-ﬁeld”part of the problem,and two more for the interaction of the outcrop-ping dislocation with rigid strip foundations.(iii)One of these analysis methods is utilised in conducting a short parametric study on the interaction of typical residential structures with a normal fault rupture.The problem studied in this paper is portrayed in Fig.1.It refers to a uniform cohesionless soil deposit of thickness H at the base of which a dip-slip fault,dipping at angle a(measured from the horizontal),produces downward or upward displacement,of vertical component h.The offset(i.e.,the differential displacement)is applied to the right part of the model quasi-statically in small consecutive steps.123hx O:“f o c u s ”O ’:“e p i c e n t e r ”Hanging wallFootwallyLW –LW hx O:“fo c u s ”O ’:“e p i c e n t e r ”Hanging wallFootwallyL W –LWq BStrip Foundation s(a )(b)Fig.1Deﬁnition and geometry of the studied problem:(a )Propagation of the fault rupture in the free ﬁeld,and (b )Interaction with strip foundation of width B subjected to uniform load q .The left edge of the foundation is at distance s from the free-ﬁeld fault outcrop2Centrifuge model testingA series of centrifuge model tests have been conducted in the beam centrifuge of the University of Dundee (Fig.2a)to investigate fault rupture propagation through sand and its in-teraction with strip footings (Bransby et al.2008a ,b ).The tests modelled soil deposits of depth H ranging from 15to 25m.They were conducted at accelerations ranging from 50to 115g.A special apparatus was developed in the University of Dundee to simulate normal and reverse faulting.A central guidance system and three aluminum wedges were installed to impose displacement at the desired dip angle.Two hydraulic actuators were used to push on the side of a split shear box (Fig.2a)up or down,simulating reverse or normal faulting,respectively.The apparatus was installed in one of the University of Dundee’s centrifuge strongboxes (Fig.2b).The strongbox contains a front and a back transparent Perspex plate,through which the models are monitored in ﬂight.More details on the experimental setup can be found in Bransby et al.(2008a ).Displacements (vertical and horizontal)at different123Fig.2(a)The geotechnicalcentrifuge of the University ofDundee;(b)the apparatus for theexperimental simulation of faultrupture propagation through sandpositions within the soil specimen were computed through the analysis of a series of digital images captured as faulting progressed using the Geo-PIV software(White et al.2003).Soil specimens were prepared within the split box apparatus by pluviating dry Fontainebleau sand from a speciﬁc height with controllable massﬂow rate.Dry sand samples were prepared at relative densities of60%.Fontainebleau sand was used so that previously published laboratory element test data(e.g Gaudin2002)could be used to select drained soil parameters for theﬁnite element analyses.The experimental simulation was conducted in two steps.First,fault rupture propagation though soil was modelled in the absence of a structure(Fig.1a),representing the free-ﬁeld part of the problem.Then,strip foundations were placed at a pre-speciﬁed distance s from the free-ﬁeld fault outcrop(Fig.1b),and new tests were conducted to simulate the interaction of the fault rupture with strip foundations.3Methods of numerical analysisThree different numerical analysis approaches were developed,calibrated,and tested.Three different numerical codes were used,in combination with soil constitutive models ranging from simpliﬁed to more sophisticated.This way,three methods were developed,each one corresponding to a different level of sophistication:(a)Method1,using the commercial FE code PLAXIS(2006),in combination with a simplenon-associated elastic-perfectly plastic Mohr-Coulomb constitutive model for soil; 123Foundation : 2-D Elastic Solid Elements Elastic BeamElementsInterfaceElements hFig.3Method 1(Plaxis)ﬁnite element diecretisation(b)Method 2,utilising the commercial FE code ABAQUS (2004),combined with a modiﬁedMohr-Coulomb constitutive soil model taking account of strain softening;and(c)Method 3,making use of the FE code DYNAFLOW (Prevost 1981),along with thesophisticated multi-yield constitutive model of Prevost (1989,1993).Centrifuge model tests that were conducted in the University of Dundee were used to validate the effectiveness of the three different numerical methodologies.The main features,the soil constitutive models,and the calibration procedure for each one of the three analysis methodologies are discussed in the following sections.3.1Method 13.1.1Finite element modeling approachThe ﬁrst method uses PLAXIS (2006),a commercial geotechnical FE code,capable of 2D plane strain,plane stress,or axisymmetric analyses.As shown in Fig.3,the ﬁnite element mesh consists of 6-node triangular plane strain elements.The characteristic length of the elements was reduced below the footing and in the region where the fault rapture is expected to propagate.Since a remeshing technique (probably the best approach when dealing with large deformation problems)is not available in PLAXIS ,at the base of the model and near the fault starting point,larger elements were introduced to avoid numerical inaccuracies and instability caused by ill conditioning of the element geometry during the displacement application (i.e.node overlapping and element distortion).The foundation system was modeled using a two-layer compound system,consisting of (see Fig.3):•The footing itself,discretised by very stiff 2D elements with linear elastic behaviour.The pressure applied by the overlying building structure has been imposed to the models through the self weight of the foundation elements.123Fig.4Method1:Calibration of constitutive model parameters utilising the FE code Tochnog;(a)oedometer test;(b)Triaxial test,p=90kPa•Beam elements attached to the nodes at the bottom of the foundation,with stiffness para-meters lower than those of the footing to avoid a major stiffness discontinuity between the underlying soil and the foundation structure.•The beam elements are connected to soil elements through an interface with a purely frictional behaviour and the same friction angleϕwith the soil.The interface has a tension cut-off,which causes a gap to develop between soil and foundation in case of detachment. Due to the large imposed displacement reached during the centrifuge tests(more than3m in several cases),with a relative displacement of the order of10%of the modeled soil height, the large displacement Lagrangian description was adopted.After an initial phase in which the geostatic stresses were allowed to develop,the fault displacement has been monotonically imposed both on the right side and the right bottom boundaries,while the remaining boundaries of the model have beenﬁxed in the direction perpendicular to the side(Fig.3),so as to reproduce the centrifuge test boundary conditions.3.1.2Soil constitutive model and calibrationThe constitutive model adopted for all of the analyses is the standard Mohr-Coulomb for-mulation implemented in PLAXIS.The calibration of the elastic and strength parameters of the soil had been conducted during the earlier phases of the project by means of the FEM code Tochnog(see the developer’s home page ),adopting a rather reﬁned and user-deﬁned constitutive model for sand.This model was calibrated with a set of experimental data available on Fontainebleau sand(Gaudin2002).Oedometer tests (Fig.4a)and drained triaxial compression tests(Fig.4b)have been simulated,and sand model parameters were calibrated to reproduce the experimental results.The user-deﬁned model implemented in Tochnog included a yielding function at the critical state,which corresponds to the Mohr-Coulomb failure criterion.A subset of those parameters was then utilised in the analysis conducted using the simpler Mohr-Coulomb model of PLAXIS:•Angle of frictionϕ=37◦•Young’s Modulus E=675MPa•Poisson’s ratioν=0.35•Angle of Dilationψ=0◦123hFoundation : Elastic Beam ElementsGap Elements Fig.5Method 2(Abaqus)ﬁnite element diecretisationThe assumption of ψ=0and ν=0.35,although not intuitively reasonable,was proven to provide the best ﬁt to experimental data,both for normal and reverse faulting.3.2Method 23.2.1Finite element modeling approachThe FE mesh used for the analyses is depicted in Fig.5(for the reverse fault case).The soil is now modelled with quadrilateral plane strain elements of width d FE =1m.The foun-dation,of width B ,is modelled with beam elements.It is placed on top of the soil model and connected through special contact (gap)elements.Such elements are inﬁnitely stiff in compression,but offer no resistance in tension.In shear,their behaviour follows Coulomb’s friction law.3.2.2Soil constitutive modelEarlier studies have shown that soil behaviour after failure plays a major role in problems related to shear-band formation (Bray 1990;Bray et al.1994a ,b ).Relatively simple elasto-plastic constitutive models,with Mohr-Coulomb failure criterion,in combination with strain softening have been shown to be effective in the simulation of fault rupture propagation through soil (Roth et al.1981,1982;Loukidis 1999;Erickson et al.2001),as well as for modelling the failure of embankments and slopes (Potts et al.1990,1997).In this study,we apply a similar elastoplastic constitutive model with Mohr-Coulomb failure criterion and isotropic strain softening (Anastasopoulos 2005).Softening is introduced by reducing the mobilised friction angle ϕmob and the mobilised dilation angle ψmob with the increase of plastic octahedral shear strain:123ϕmob=ϕp−ϕp−ϕresγP fγP oct,for0≤γP oct<γP fϕres,forγP oct≥γP f(1)ψmob=⎧⎨⎩ψp1−γP octγP f,for0≤γP oct<γP fψres,forγP oct≥γP f⎫⎬⎭(2)whereϕp andϕres the ultimate mobilised friction angle and its residual value;ψp the ultimate dilation angle;γP f the plastic octahedral shear strain at the end of softening.3.2.3Constitutive model calibrationConstitutive model parameters are calibrated through the results of direct shear tests.Soil response can be divided in four characteristic phases(Anastasopoulos et al.2007):(a)Quasi-elastic behavior:The soil deforms quasi-elastically(Jewell and Roth1987),upto a horizontal displacementδx y.(b)Plastic behavior:The soil enters the plastic region and dilates,reaching peak conditionsat horizontal displacementδx p.(c)Softening behavior:Right after the peak,a single horizontal shear band develops(Jewelland Roth1987;Gerolymos et al.2007).(d)Residual behavior:Softening is completed at horizontal displacementδx f(δy/δx≈0).Then,deformation is accumulated along the developed shear band.Quasi-elastic behaviour is modelled as linear elastic,with secant modulus G S linearly incre-asing with depth:G S=τyγy(3)whereτy andγy:the shear stress and strain atﬁrst yield,directly measured from test data.After peak conditions are reached,it is assumed that plastic shear deformation takes placewithin the shear band,while the rest of the specimen remains elastic(Shibuya et al.1997).Scale effects have been shown to play a major role in shear localisation problems(Stone andMuir Wood1992;Muir Wood and Stone1994;Muir Wood2002).Given the unavoidableshortcomings of the FE method,an approximate simpliﬁed scaling method(Anastasopouloset al.2007)is employed.The constitutive model was encoded in the FE code ABAQUS(2004).Its capability toreproduce soil behaviour has been validated through a series of FE simulations of the directshear test(Anastasopoulos2005).Figure6depicts the results of such a simulation of denseFontainebleau sand(D r≈80%),and its comparison with experimental data by Gaudin (2002).Despite its simplicity and(perhaps)lack of generality,the employed constitutivemodel captures the predominant mode of deformation of the problem studied herein,provi-ding a reasonable simpliﬁcation of complex soil behaviour.3.3Method33.3.1Finite element modeling approachTheﬁnite element model used for the analyses is shown for the normal fault case in Fig.7.The soil is modeled with square,quadrilateral,plane strain elements,of width d FE=0.5m. 123Fig.6Method 2:Calibration ofconstitutive model—comparisonbetween laboratory direct sheartests on Fontainebleau sand(Gaudin 2002)and the results ofthe constitutive modelx D v3.3.2Soil constitutive ModelThe constitutive model is the multi-yield constitutive model developed by Prevost (1989,1993).It is a kinematic hardening model,based on a relatively simple plasticity theory (Prevost 1985)and is applicable to both cohesive and cohesionless soils.The concept of a “ﬁeld of work-hardening moduli”(Iwan 1967;Mróz 1967;Prevost 1977),is used by deﬁning a collection f 0,f 1,...,f n of nested yield surfaces in the stress space.V on Mises type surfaces are employed for cohesive materials,and Drucker-Prager/Mohr-Coulomb type surfaces are employed for frictional materials (sands).The yield surfaces deﬁne regions of constant shear moduli in the stress space,and in this manner the model discretises the smooth elastic-plastic stress–strain curve into n linear segments.The outermost surface f n represents a failure surface.In addition,accounting for experimental evidence from tests on frictional materials (de 1987),a non-associative plastic ﬂow rule is used for the dilatational component of the plastic potential.Finally,the material hysteretic behavior and shear stress-induced anisotropic effects are simulated by a kinematic rule .Upon contact,the yield surfaces are translated in the stress space by the stress point,and the direction of translation is selected such that the yield surfaces do not overlap,but remain tangent to each other at the stress point.3.3.3Constitutive model parametersThe required constitutive parameters of the multi-yield constitutive soil model are summari-sed as follows (Popescu and Prevost 1995):a.Initial state parameters :mass density of the solid phase ρs ,and for the case of porous saturated media,porosity n w and permeability k .b.Low strain elastic parameters :low strain moduli G 0and B 0.The dependence of the moduli on the mean effective normal stress p ,is assumed to be of the following form:G =G 0 p p 0 n B =B 0 p p 0n (4)and is accounted for,by introducing two more parameters:the power exponent n and the reference effective mean normal stress p 0.c.Yield and failure parameters :these parameters describe the position a i ,size M i and plastic modulus H i ,corresponding to each yield surface f i ,i =0,1,...n .For the case of pressure sensitive materials,a modiﬁed hyperbolic expression proposed by Prevost (1989)and Grifﬁths and Prévost (1990)is used to simulate soil stress–strain relations.The necessary parameters are:(i)the initial gradient,given by the small strain shear modulus G 0,and (ii)the stress (function of the friction angle at failure ϕand the stress path)and strain,εmax de v ,levels at failure.Hayashi et al.(1992)improved the modiﬁed hyperbolic model by introducing a new parameter—a —depending on the maximum grain size D max and uniformity coefﬁcient C u .Finally,the coefﬁcient of lateral stress K 0is necessary to evaluate the initial positions a i of the yield surfaces.d.Dilation parameters :these are used to evaluate the volumetric part of the plastic potentialand consist of:(i)the dilation (or phase transformation)angle ¯ϕ,and (ii)the dilation parameter X pp ,which is the scale parameter for the plastic dilation,and depends basically on relative density and sand type (fabric,grain size).With the exception of the dilation parameter,all the required constitutive model parameters are traditional soil properties,and can be derived from the results of conventional laboratory 123Table1Constitutive model parameters used in method3Number of yield surfaces20Power exponent n0.5Shear modulus G at stress p1 (kPa)75,000Bulk modulus at stress p1(kPa)200,000Unit massρ(t.m−3) 1.63Cohesion0 Reference mean normal stressp1(kPa)100Lateral stress coefﬁcient(K0)0.5Dilation angle in compression (◦)31Dilation angle in extension(◦)31Ultimate friction angle in compression(◦)41.8Ultimate friction angle inextension(◦)41.8Dilation parameter X pp 1.65Max shear strain incompression0.08Max shear strain in extension0.08Generation coefﬁcient in compressionαc 0.098Generation coefﬁcient inextensionαe0.095Generation coefﬁcient in compressionαlc 0.66Generation coefﬁcient inextensionαle0.66Generation coefﬁcient in compressionαuc 1.16Generation coefﬁcient inextensionαue1.16(e.g.triaxial,simple shear)and in situ(e.g.cone penetration,standard penetration,wave velocity)soil tests.The dilational parameter can be evaluated on the basis of results of liquefaction strength analysis,when available;further details can be found in Popescu and Prevost(1995)and Popescu(1995).Since in the present study the sand material is dry,the cohesionless material was modeled as a one-phase material.Therefore neither the soil porosity,n w,nor the permeability,k,are needed.For the shear stress–strain curve generation,given the maximum shear modulus G1,the maximum shear stressτmax and the maximum shear strainγmax,the following functional relationship has been chosen:For y=τ/τmax and x=γ/γr,withγr=τmax/G1,then:y=exp(−ax)f(x,x l)+(1−exp(−ax))f(x,x u)where:f(x,x i)=(2x/x i+1)x i−1/(2x/x i+1)x i+1(5)where a,x l and x u are material parameters.For further details,the reader is referred to Hayashi et al.(1992).The constitutive model is implemented in the computer code DYNAFLOW(Prevost1981) that has been used for the numerical analyses.3.3.4Calibration of model constitutive parametersTo calibrate the values of the constitutive parameters,numerical triaxial tests were simulated with DYNAFLOW at three different conﬁning pressures(30,60,90kPa)and compared with the results of available physical tests conducted on the same material at the same conﬁning pressures.The parameters are deﬁned based on the shear stress versus axial strain curve and volumetric strain versus axial strain curve.Figure8illustrates the comparisons between numerical simulations and physical tests in terms of volumetric strain and shear stress versus123Table2Summary of main attributes of the centrifuge model testsTest Faulting B(m)q(kPa)s(m)g-Level a D r(%)H(m)L(m)W(m)h max(m) 12Normal Free—ﬁeld11560.224.775.723.53.1528Reverse Free—ﬁeld11560.815.175.723.52.5914Normal10912.911562.524.675.723.52.4929Reverse10919.211564.115.175.723.53.30a Centrifugal accelerationFig.9Test12—Free-ﬁeld faultD r=60%Fontainebleau sand(α=60◦):Comparison ofnumerical with experimentalvertical displacement of thesurface for bedrock dislocationh=3.0m(Method1)and2.5m(Method2)[all displacements aregiven in prototype scale]Structure Interaction(FR-SFSI):(i)Test14,normal faulting at60◦;and(ii)Test29,reverse faulting at60◦.In this case,the comparison is conducted for all of the developed numerical analysis approaches.The main attributes of the four centrifuge model tests used for the comparisons are syn-opsised in Table2,while more details can be found in Bransby et al.(2008a,b).4.1Free-ﬁeld fault rupture propagation4.1.1Test12—normal60◦This test was conducted at115g on medium-loose(D r=60%)Fontainebleau sand,simu-lating normal fault rupture propagation through an H=25m soil deposit.The comparison between analytical predictions and experimental data is depicted in Fig.9in terms of vertical displacement y at the ground surface.All displacements are given in prototype scale.While the analytical prediction of Method1is compared with test data for h=3.0m,in the case of Method2the comparison is conducted at slightly lower imposed bedrock displacement: h=2.5m.This is due to the fact that the numerical analysis with Method2was conducted without knowing the test results,and at that time it had been agreed to set the maximum displacement equal to h max=2.5m.However,when test results were publicised,the actually attained maximum displacement was larger,something that was taken into account in the analyses with Method1.As illustrated in Fig.9,Method2predicts almost correctly the location of fault out-cropping,at about—10m from the“epicenter”,with discrepancies limited to1or2m.The deformation can be seen to be slightly more localised in the centrifuge test,but the comparison between analytical and experimental shear zone thickness is quite satisfactory.The vertical displacement proﬁle predicted by Method1is also qualitatively acceptable.However,the123Method 2Centrifuge Model TestR1S1Method 1(a )(b)(c)Fig.10Test 12—-Normal free-ﬁeld fault rupture propagation through H =25m D r =60%Fontainebleau sand:Comparison of (a )Centrifuge model test image,compared to FE deformed mesh with shear strain contours of Method 1(b ),and Method 2(c ),for h =2.5mlocation of fault rupture emergence is a few meters to the left compared with the experimen-tal:at about 15m from the “epicenter”(instead of about 10m).In addition,the deformation predicted by Method 1at the ground surface computed using method 1is widespread,instead of localised at a narrow band.FE deformed meshes with superimposed shear strain contours are compared with an image from the experiment in Fig.10,for h =2.5m.In the case of Method 2,the comparison can be seen to be quite satisfactory.However,it is noted that the secondary rupture (S 1)that forms in the experiment to the right of the main shear plane (R 1)is not predicted by Method 2.Also,experimental shear strain contours (not shown herein)are a little more diffuse than the FE prediction.Overall,the comparison is quite satisfactory.In the case of Method 1,the quantitative details are not in satisfactory agreement,but the calculation reveals a secondary rupture to the right of the main shear zone,consistent with the experimental image.4.1.2Test 28—reverse 60◦This test was also conducted at 115g and the sand was of practically the same relative density (D r =61%).Given that reverse fault ruptures require larger normalised bedrock123Fig.11Test28—Reversepropagation through H=15mD r=60%Fontainebleau sand:Comparison of numerical withexperimental verticaldisplacement of the surface forbedrock dislocation h=2.0m(all displacements are given inprototype scale)displacement h/H to propagate all the way to the surface(e.g.Cole and Lade1984;Lade et al.1984;Anastasopoulos et al.2007;Bransby et al.2008b),the soil depth was set at H=15m.This way,a larger h/H could be achieved with the same actuator.Figure11compares the vertical displacement y at the ground surface predicted by the numerical analysis to experimental data,for h=2.0m.This time,both models predict correctly the location of fault outcropping(deﬁned as the point where the steepest gradient is observed).In particular,Method1achieves a slightly better prediction of the outcropping location:−10m from the epicentre(i.e.,a difference of1m only,to the other direction). Method2predicts the fault outbreak at about−7m from the“epicenter”,as opposed to about −9m of the centrifuge model test(i.e.,a discrepancy of about2m).Figure12compares FE deformed meshes with superimposed shear strain contours with an image from the experiment,for h=2.5m.In the case of Method2,the numerical analysis seems to predict a distinct fault scarp,with most of the deformation localised within it.In contrast,the localisation in the experiment is clearly more intense,but the fault scarp at the surface is much less pronounced:the deformation is widespread over a larger area.The analysis with Method1is successful in terms of the outcropping location.However,instead of a single rupture,it predicts the development of two main ruptures(R1and R2),accompanied by a third shear plane in between.Although such soil response has also been demonstrated by other researchers(e.g.Loukidis and Bouckovalas2001),in this case the predicted multiple rupture planes are not consistent with experimental results.4.2Interaction with strip footingsHaving validated the effectiveness of the developed numerical analysis methodologies in simulating fault rupture propagation in the free-ﬁeld,we proceed to the comparisons of experiments with strip foundations:one for normal(Test14),and one for reverse(Test29) faulting.This time,the comparison is extended to all three methods.4.2.1Test14—normal60◦This test is practically the same with the free-ﬁeld Test12,with the only difference being the presence of a B=10m strip foundation subjected to a bearing pressure q=90kPa.The foundation is positioned so that the free-ﬁeld fault rupture would emerge at distance s=2.9m from the left edge of the foundation.123。

对比研究报告模板英文

Comparative Research Report Template AbstractThe purpose of this comparative research report template is to provide researchers with a clear structure for creating a comprehensive and effective research report. The template includes sections for the introduction, background, methodology, results, analysis, and conclusion. By following this template, researchers can ensure that their reports are both well-organized and easy to read.IntroductionThe introduction should provide a brief overview of the topic being studied and emphasize why the research is important. It should also include research questions and objectives, as well as a clear statement of the hypothesis being tested. In this section, it is paramount to provide context and significance of the research problem.BackgroundIn this section, the researcher must provide a comprehensive and thorough review of the literature on the study topic. The literature review should summarize previous research studies, theories, and notable works in the field. The literature review should be an overview of the background information relevant to the research, which may include historical background, current contexts, phenomena or theories related to the research. The literature review is crucial in establishing the need for and importance of the research, as well as highlight gaps in knowledge presenting the opportunity for further research.MethodologyThe methodology section should detail how the research was conducted, including participants, data collection instruments, and procedures. This section should be clear and concise to ensure the reproducibility of the study. It should also include a discussion of the reliability and validity of the data and the analysis methods employed in the research. In addition, this section should also include a statement of ethics, including the protection of human subjects in research.ResultsIn this section, the researcher presents the findings of the research. This may include qualitative or quantitative data analysis. The presentation of the results should be clear, concise and make effective use of tables, graphs and charts. The statistical analysis used to analyze data should be clearly explained to ensure readers can understand how the data were analyzed. The interpretation of theresults should be unbiased, and a discussion of any limitations of the research should also be presented.AnalysisIn this section, the researcher analyzes the results and discusses how they relate to the research questions and objectives. The researcher should consider the study’s implications, identify significant points, patterns in the data, and compare results with previous research in the field. This section should include a discussion of the implications of the research and the conclusions that can be drawn.ConclusionThe conclusion is a brief summary of the main findings of the research study. It should include a statement of the study’s contributions to the field and present recommendations for future research. Also, the researcher must draw inferences, consolidate observations and offer a firm conclusion to the research problem discussed in the introduction.ReferencesFinally, it is essential to provide the sources that were used to inform the report. The references should follow in the selected citation style. The references should be comprehensive and should include all the sources that were cited in the report.。

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Comparison and Evaluation Between MODIS Vegetation Indices in Northwest ChinaNi Guo,Xiaoping Wang, Dihua Cai, Jia YangInstitute of Arid Meteorology, CMA,Lanzhou; Key Laboratory of Arid Climatic Change and Reducing Disaster of Gansu Province; Key Open Laboratory of Arid Climate Change and Disaster Reduction of CMA, Lanzhou, 730000,China.E-mail: guoni0531@Abstract —The Moderate Resolution Imaging Spectroradiometer (MODIS) VI products provide consistent, spatial, and temporal comparisons of global vegetation conditions that can be used to monitor the Earth's terrestrial photosynthetic activity. In order to compare and evaluate the ability that the MODIS two indices （VI ），the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) ，monitor vegetation over a diverse range of biomes in Northwest China, the MODIS vegetation index products data set were used, which are from NASA LP DAAC(Land Process Distributed Active Archive Center) and have 250m resolution and 16-day composting periods in 2004.Different vegetation types including broadleaf forests, needleleaf forests, meadows, grassland, steppes, scrubs, desert and cultivated vegetation were chosen as representative types, based on Vegetation Map of Northwest China. Two CE-313 radiancements, which is portable instrument and has five filters between 450 and 1650 nm, were used to collect the spectrum data for calculating vegetation indices over different vegetation types in the plants growth season. The levels of vegetation cover were observed at the same spots and the same time when the CE-313 radiancements worked.The results show that the NDVI was higher than the EVI in most part of Northwest China. The difference between the two indices increased from deserts, steppes, cultivated vegetation, meadows to forest. Both NDVI and EVI were well indicated the distribution and the growth of various vegetations in arid and semi-arid area. The NDVI saturated in these high biomass types vegetations, such as broadleaf forests, needleleaf forests, meadows and part of cultivated vegetation, during the plants blooming period. The NDVI did not increase with the growth of these vegetations during the period. The EVI was different. It increased with the growth of vegetation. The NDVI saturation threshold was about 0.8. The length of the period that NDVI saturating was different from 1 to 5 months with the type of vegetation. The length of the period of broadleaf forests was the longest and that of the cultivated vegetation was the shortest. Though the height of high-cold meadows was not higher than forty cm in northeast of Tibetan Plateau, the NDVI of these regions saturated for 2 months during the plants flourished.The data gotten from the CE-313 showed that the NDVI of many kinds vegetation, for example, winter wheat, corn, clover blossom and forb high cold meadows, may saturate during the process of becoming mature. Crossplots of the two VIs showed a curvilinear relationship between them, such that the NDVI always had higher values but appeared to reach an asymptotic maximum value. When the NDVI was about 0.8, the NDVI was almost stop responding with the vegetation density while the EVI was still responding. Both NDVI and EVI have a good linearcorrelation with the levels of vegetation cover according to thedata gotten from the CE-313. The Correlation of EVI was better than that of NDVI especially in the higher levels vegetation cover. The Correlation coefficient of EVI was 0.8112 and the NVDI was 0.6946.Keywords- MODIS vegetation indices; CE313;Northwest China;NDVI;EVII. I NTRODUCTIONNorthwest China lies in the inland of Eurasia, which is the interjection place of Tibetan plateau, loess plateau and Mengxin plateau[1], containing many kinds of land cover types, such as gobi desert, desert, glacier, snow cover, lake, river, forest, grassland, cropland etc. Due to the influences of several synoptic systems, such as monsoon weather system, westerly weather system, plateau weather system[2], and complicated geography conditions, the climate in Northwest exhibits complexity and multiplicity. Annual precipitation varies from more than 1000 mm in Qinling Mountain to less than 20 mm in the extremity arid area of the Sinkiang Uigur Autonomous Region[3]. Climatic zones involve north sub-tropical zone, warm zone, plateau cold and sub-cold zone climate regions including humid, semi-humid, semi-arid, arid, and extremely arid regions[4]. The vegetation coverage and vegetation species are quite different in these regions, such as broadleaf deciduous forest, needleleaf forests, scrubs, meadows, steppes, deserts etc[5]. East of Northwest is semi-arid to semi-humid rain-fed cropland region, where the precipitation is in the range of 300-600 mm, and vegetation growth mostly depends on natural precipitation. West of Northwest belongs to arid or extremely arid region, where distribute boundless deserts and gobi deserts. Owing to the vapor is held up by a series of high mountains, such as Qilian Mountain, Kunlun Mountain, Tian Mountain, a number of glaciers and snow cover be produced on the mountains, which lead to form a lot of inland rivers and thereby a number of oasis be formed. Northeast of Tibetan plateau which locats in south of Northwest with the mean elevation of 4000 m is the headstream of Yangtse River, Yellow River and Lancang River, and there is the best pastures with the highest altitute in the world. However, with the increase of population and changes of climate in the past 5 decades, many eco-problems, such as drought, grassland degradation, desertification, river break, and lake atrophy etc. [6] [7], were raised in Northwest of china. Remote sensing is the one of the most effective methods monitoring changes of the ecosystem in these areas.The Moderate Resolution Imaging Spectroradiometer (MODIS) VI products provide consistent, spatial, and temporal comparisons of global vegetation conditions that can be used to monitor the Earth's terrestrial photosynthetic activity [8]. One vegetation index is the normalized difference vegetation index (NDVI) that is referred to asSupported by The National Natural Science Foundation of China Project( No. 40375011)the “continuity index” to the existing 20+-year NOAA AVHRR-derived NDVI time series, which could be extended by MODIS data to provide a long-term data record for use in operational monitoring studies. The “enhanced” vegetation index (EVI) improves sensitivity over high biomass regions and improves on vegetation monitoring capability through a decoupling of the canopy background signal and a reduction in atmosphere influences [9].In order to compare and evaluate the ability of the NDV and EVI monitoring vegetation over a diverse range of biomes in Northwest China, the MODIS vegetation index products data ，the filed vegetation spectrum data and the vegetation coverage(%) were used. The relationships between the two VIs and vegetation coverage(%) were analysis.II.S TUAY A REA AND M ETHODSA.Study AreaThe study area locates in the middle-east of Northwest China including 4 provinces, Gansu, Qinghai, Ningxia and Shanxi. The longitude is from 92˚E to 112˚E and the latitude is from 31˚N to 42˚N. Three sites, semi-humid climate region, semi-arid climate region, and high-cold and humid plateau climate region, were chosen to carry out filed plants spectrum observation. The site-1 is located in Dongzhiyuan， Qingyang Region of Gansu Province, a semi-humid rain-fed cropland, with a mean annual temperature of 9.6 o C, a mean annual precipitation of 556 mm, and the elevation of 1335 m. The center of the site is 107˚38.655' E，35˚35.897' N, which observational range is about 1 km2. The observational period was from April to May 2004, and the observed plants were winter wheat, cole, clover and corn. The site-2 is in Wangliangfu, Dinfxi of Gansu Province, a semi-arid rain-fed cropland, where the mean annual temperature is 6.7 o C, the mean annual precipitation is 397.9 mm, and the elevation is 1896.7 m. The center of the site is 104˚36.37' E, 35˚35.60' N, which observational area was approximately 0.6 km2. The observational period was in 10-17 June 2004, and the observed plants were spring wheat and benne. The site-3 is in Maqu high-cold meadows, Gannan of Gansu Province, with the mean annual temperature of 1.6 o C, the mean annual precipitation of 591.2 mm and the elevation of 3470 m. The observational area was about 2 km2 and the center of the site is about 101º51.166' E，34º02.275' N. The observational period was in 20-30 August 2004. The observed plants were Kobresia.Figure 1. Study area Sketch Map （a.oasis，b.Steppes，c.High-cold Meadows，d.Scrubs，e.Deserts，f.Broadleaf Forests，g. Rain-fed Cropland，h.Needleaf Forests.filed observation sites）B. Data and MethodsMODIS vegetation indices data is the MODIS product(MOD13Q1) which is at 16-day intervals and 250m resulation，providing by NASA LP DAAC (The Land Processes DistributedActive Archive Center ) .The tilcs are h25v05、h26v05、h27v06 and h25v04.The data period is from 1 Jan. 2004 to 2 Jan. 2005. The lower quality pixels were removed using QA quality analysis. Composting two of the 16 days data using maximum value composite, in which the number of days in the month was the most, got the monthly data.Several vegetation types including broadleaf forests, need leaf forests, high-cold meadows, scrubs, steppes, deserts, rain-fed croplands, oasis croplands were selected as typical sample areas according to Vegetation Atlas of China[5]. The NDVI and EVI of each typical area were exported from monthly VIs data.Two CE 313 radiancements made by CIMEL Company in Franc were used to get vegetation canopy spectral at the three filed sites. The radiancements have 10º FOV and have 5 bands between 450 and 1650 nm and the bandwidth is 428nm～472nm, 514nm～584nm, 621nm～671nm, and 787nm～883nm and 1555nm～1699 nm respectively. The data are stored in a memory and can be easily transferred in a ASCII file on a PC .The sensor of one instrument was mounted on a frame looking nadir from a high of 3m to observe objectives while the other aimed at a standard spectral reference panel continuously at a high of 0.5m.The interval among the objective spots was five meters with five repeats at every spot. Percent vegetation coverage(%) ,GPS location and digital photo were gotten at every spot at same time. average value of objective divided by standard panel could offer the canopy reference .The NDVI and EVI can be calculated via the following equations.Re Re()/()Nir d Nir dNDVIρρρρ=−+（1）Re1Re22.5()/()Nir d Nir d Blue EVI C C Lρρρρρ=×−+×−×+ (2) Where ρNir, ρBlue and ρred are the surface reflectance in their respective sensor bands; L is a canopy background adjustment that addresses nonlinear differential NIR and red radiant transfer through a canopy, C1 and C2 and are the coefficients of the aerosol term, which uses the blue band to correct for aerosols in the red band. We used the coefficients adopted in the MODIS EVI standard product L=1,C1=6 and C2=7.5 [9].The statistical relationships between the two VIs and vegetation coverage(%) were analysis. The coverage involve all of coverage, high coverage, middle coverage and low coverageIII.R ESULTSA. Seasonal Charateristics of Various VegetationsFig.2 shows the MODIS NDVI and EVI seasonal profiles of various vegetations. It can be found that both NDVI and EVI had significant seasonal variations in many types of vegetations，such as broadleaf forests, need leaf forests, high-cold meadows, rain-fed cropland, oasis cropland. The VIs profiles of rain-fed croplands in Guanzhong of Shanxi Province had double peaks while others had single peak. The values of VIs in deserts were very small and show very slight fluctuations. NDVI of these vegetations varied in the range of 0.039-0.847, while EVI in the range of 0.029-0.634 were slightly lower than the NDVI. Both NDVI and EVI had similar seasonalprofiles depicted the growth of the various biomes fairly well. The NDVI were always higher than EVI, and the difference of the two VIs increased in the order of deserts, steeps, croplands and forests withNDVI ascending.Figure 2.The MODIS NDVI and EVI seasonal profiles in2004B. The Relationship Between MODIS NDVI and EVIMany studies show that NDVI saturate over high biomes[10].The NDVI of forests showed obvious saturation in Fig.2. Fig.3 is the Crossplots of the MODIS NDVI and EVI on August 2004, which showed that NDVI always had higher values than EVI and appeared to reach an asymptotic maximum value. The NDVI showed higher increase than EVI when it was less than 0.7. NDVI ascended from 0.087 to 0.9, while EVI from 0.074 to 0.82. When NDVI were more than 0.8, it presented a slight variation from 0.8 to 0.9, which indicated that it saturated, but EVI varied from 0.45-0.82. It was exhibited that the NDVI over Broadleaf forests, need leaf forests, high-cold meadows, rain-fed cropland saturated.Figure 3. The Crossplots of the MODIS NDVI and EVI for several types ofvegetation on August 2004Figure 4. The Crossplots of the MODIS NDVI and EVI over forests in 2004.From the Crossplots of the MODIS NDVI and EVI over forests for the whole year 2004 (F ig.4), it could be seen that the NDVI of broadleaf forests slightly varied around 0.8 during May and Oct., while EVI varied from 0.4 to 0.8. It could be know that the NDVI of needleleaf forests slightly varied from June to Sep.，but the EVI increased obviously during same period. The NDVI was saturating and almost stop responding with the vegetation growth while the EVI was still responding well. It was been found that the length of the period that NDVI saturating was different from 1 to 5 months with the type of vegetation. The length of the saturating period of broadleaf forests was 5 months and that of the croplands was one month. Though the height of high-cold meadows was not higher than one meter in northeast of Tibetan Plateau, the NDVI of these regions saturated for 2 months during the plants flourished.C. The Relationship Between NDVI and EVI From Vegetation SpectrumFig. 5 is the crossplots of the NDVI and EVI over various vegetations and of different vegetation coverages (%) gotten from the three filed observation sites. It showed that NDVI saturated during these periods those high-cold meadows was in flowering period clover in flower bud appearing period and wheat in flowering period （left Fig5）. The NDVI was going to flat and did not responding to the rise of biomass, but the EVI increased continuously. The threshed of the NDVI saturated was about 0.8. It could also be seen that when the coverage (%) was greater than 70％, the NDVI saturated, which rose slowly with the coverage, however, the EVI did not. The saturate NDVI over high biomass can bring some errors in estimating the vegetation growth capability, fortunately EVI solved the problem, which do not saturate in high vegetation coverage area and can improve the precision monitoring globe vegetation.Figure 5. The crossplots of the NDVI and EVI over various vegetations andof different vegetation coverages (%)D.The statistical relationship of the VIs and coverage (%)The statistical analyses of relationship of the NDVI and EVI calculated based on the field observation spectrum data with the coverage (%) from filed sites showed that there were an obviousFigure 6. The correlation between the two VIs and coverage (%)liner correlation between the two VIs and coverage (%). The R2 of EVI and NDVI with coverage was 0.8592 and 0.7943 respectively, which indicated that the correlation of EVI with coverage was better than that of the NDVI for all kinds of coverages. The R2 of EVI with coverage, 0.658, was much more than that of NDVI, 0.4565, in high level coverage. The R2 of EVI and NDVI with coverage were 0.4825 and 0.5057 in middle level coverage, and 0.5588 and 0.601 in low level coverage, respectively.Huete et al found that canopy background noise is most severe at intermediate levels of vegetation (40–60% coverage) [8]. EVI inheriting SAVI (soil-adjusted vegetation index) have better performance at decreasing background noise. The reasons that the EVI did not have better correlation with coverage than the NDVI at the middle and low coverage are not understand. It needs to do more works in future.IV.C ONCLUSIONThe study showed that both NDVI and EVI had similar seasonal profiles and depicted the growth of the various biomes well in Northwest China. The NDVI were always higher than the EVI. 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